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Charges up turn one, then attacks on turn two.
Like all moves that take a turn to charge up, SolarBeam is not a good competitive move. Grass is not the best attacking type and it has access to better options to attack weak opponents, such as Mega Drain and Razor Leaf, which only take one turn. Do not use this move.
Mega Drain is a much less powerful alternative that also heals the user.
Razor Leaf is a less powerful alternative with a high critical chance.